fix backslashes in path used for `asm_tests` #732

Job	Run time
build (libgccjit.so, master)	6m 56s
build (libgccjit_without_int128.so, master-without-128bit-integers)	6m 55s
build (libgccjit12.so, gcc12, --no-default-features, TEST_FLAGS='-Cpanic=abort -Zpanic-abort-test...	1m 20s
success_failures	1s
	15m 12s

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